This patch has been proposed upstream:
https://lists.systemreboot.net/guile-email/87mtnv1r2p.fsf@gnu.org/T/#u.

From c21fe0f0e28b80b606973d3e372e2bc8528c9766 Mon Sep 17 00:00:00 2001
From: Mathieu Othacehe <othacehe@gnu.org>
Date: Wed, 29 Sep 2021 12:47:35 +0000
Subject: [PATCH 1/1] email: Do not use an empty bytevector to test the
 charset.

Using an empty bytevector no longer throws an exception since this Guile
commit: 5ea8c69e9153a970952bf6f0b32c4fad6a28e839.

* email/email.scm (post-process-content-transfer-encoding): Use a bytevector
containg the 'e' character to test the charset validity.
---
 email/email.scm | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/email/email.scm b/email/email.scm
index 3f4e194..ac70463 100644
--- a/email/email.scm
+++ b/email/email.scm
@@ -832,7 +832,8 @@ values. The returned headers is a string and body is a bytevector."
   (define (valid-charset? charset)
     (catch #t
       (lambda ()
-        (bytevector->string (make-bytevector 0 0) charset)
+        ;; Try to convert a bytevector containg the 'e' character.
+        (bytevector->string (make-bytevector 1 48) charset)
         #t)
       (const #f)))
 
-- 
2.33.0

